Study of Massive Stars Confirms Milky Way has Four Spiral Arms

New research published in the Monthly Notices of the Royal Astronomical Society claims to end the long-standing debate over whether the Milky Way galaxy has two or four "arms."

Because we are living inside the Milky Way, we are unable to see what it looks like from the outside looking in. But by carefully observing massive stars within in the galaxy, its shape can be deduced.

The latest research contends to reaffirm that the Milky Way has four spiral arms, not two.

"The Milky Way is our galactic home and studying its structure gives us a unique opportunity to understand how a very typical spiral galaxy works in terms of where stars are born and why," said Professor Melvin Hoare, a member of the RMS survey at Leeds and a co-author of the research paper.

The 12-year-long study of massive stars in the Milky Way conducted by Red MSX Source (RMS) survey led by University of Leeds astronomers scoured the galaxy for massive young stellar objects, which are uncommonly large. These massive stars lives shorter lives than their lower mass counterparts, which astronomers utilized when conducting their survey. As these massive stars die relatively quickly, they are only found in the arms of the galaxy where they formed; they have no time to move elsewhere.

Research done in the 1950s concluded that our galaxy has four spiral arms, based on the clouds of gas in which new stars form. But a later analysis by the NASA Spitzer Space Telescope that scoured the Milky Way with its infrared scope revealed about 110 million stars, but only evidence of two spiral arms.

For the latest study, a network of ground telescopes were used to examine massive stars identified by the RMS survey. About 1,650 stars were included in the observations, from which astronomers were able to calculate the distance and luminosity of the massive stars. Plotting the stars out revealed a distribution across four spiral arms.

"It isn't a case of our results being right and those from Spitzer's data being wrong - both surveys were looking for different things," Hoare said. "Spitzer only sees much cooler, lower mass stars - stars like our Sun - which are much more numerous than the massive stars that we were targeting.

"Lower mass stars live much longer than massive stars and rotate around our Galaxy many times, spreading out in the disc. The gravitational pull in the two stellar arms that Spitzer revealed is enough to pile up the majority of stars in those arms, but not in the other two," Hoare continued. "However, the gas is compressed enough in all four arms to lead to massive star formation."

Lead study author James Urquhart from the Max Planck Institute for Radio Astronomy in Bonn, Germany expressed his excitement over the confirmation that there are four spiral arms in our galaxy.

"It's exciting that we are able to use the distribution of young massive stars to probe the structure of the Milky Way and match the most intense region of star formation with a model with four spiral arms," he said.

For Hoare, the results reaffirm a notion that he grew up believing.

"Star formation researchers, like me, grew up with the idea that our Galaxy has four spiral arms. It's great that we have been able to reaffirm that picture," he said.